有没有办法可以反转并导出.m4a音频文件？我找到了一个反转音频轨道
here的解决方案,但它似乎只是在处理.caf文件格式.如果唯一的方法是使用.caf,有没有办法首先将.m4a文件转换为.caf？

更新：
在another post我发现AVAssetReader可用于从音频文件中读取音频样本,但我不知道如何以相反的顺序写回样本.以下代码片段直接来自帖子.任何帮助,将不胜感激.谢谢

+ (void) reverseAudioTrack: (AVAsset *)audioAsset outputURL: (NSURL *)outputURL {
NSError *error;

AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:audioAsset error:&error];
if (error) {NSLog(@"%@", error.localizedDescription);}

AVAssetTrack* track = [[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

NSMutableDictionary* audioReadSettings = [NSMutableDictionary dictionary];
[audioReadSettings setValue:[NSNumber numberWithInt:kAudioFormatLinearPCM]
                     forKey:AVFormatIDKey];

AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track outputSettings:audioReadSettings];
[reader addOutput:readerOutput];
[reader startReading];

CMSampleBufferRef sample; //= [readerOutput copyNextSampleBuffer];
NSMutableArray *samples = [[NSMutableArray alloc] init];

// Get all samples
while((sample = [readerOutput copyNextSampleBuffer])) {
    [samples addObject:(__bridge id)sample];
    CFRelease(sample);
}

// Process samples in reverse
AudioChannelLayout acl;
bzero(&acl, sizeof(acl));
acl.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:outputURL
                                                   fileType:AVFileTypeAppleM4A
                                                      error:&error];
if (error) {NSLog(@"%@", error.localizedDescription);}
NSDictionary *writerOutputSettings = [ NSDictionary dictionaryWithObjectsAndKeys:
                                      [ NSNumber numberWithInt: kAudioFormatAppleLossless ], AVFormatIDKey,
                                      [ NSNumber numberWithInt: 16 ], AVEncoderBitDepthHintKey,
                                      [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                      [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                      [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey, nil ];

AVAssetWriterInput *audioWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:writerOutputSettings];

[writer addInput:audioWriterInput];
[writer startWriting];
[writer startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[0]) ];

// (1) Would it work if I loop in reverse here?
for (NSInteger i = 0; i < samples.count; i++) {
    CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer((__bridge CMSampleBufferRef)samples[i]);

    CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples((__bridge CMSampleBufferRef)samples[i]);
    AudioBufferList audioBufferList;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer((__bridge CMSampleBufferRef)samples[i],
                                                            NULL,
                                                            &audioBufferList,
                                                            sizeof(audioBufferList),
                                                            NULL,
                                                            NULL,
                                                            kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
                                                            &buffer
                                                            );

    for (int bufferCount = 0; bufferCount < audioBufferList.mNumberBuffers; bufferCount++) {
        SInt16* samples = (SInt16 *)audioBufferList.mBuffers[bufferCount].mData;
        for (int i=0; i < numSamplesInBuffer; i++) {
            // amplitude for the sample is samples[i], assuming you have linear pcm to start with

            // (2) What should I be doing to write the samples into an audio file?
        }
    }
    CFRelease(buffer);
}

最佳答案 是的,有一种方法可以处理,然后导出任何有iOS支持的音频文件.

但是,大多数这些格式(mp3命名为1)都是有损和压缩的.您必须首先解压缩数据,应用转换并重新压缩.您将应用于音频信息的大多数转换应该可以在原始PCM级别完成.

结合这两个语句,您可以在几个过程中执行此操作：

>将原始文件转换为符合kAudioFormatLinearPCM的音频文件,如AIFF
>处理该临时文件(反转其内容)
>将临时文件转换回原始格式

就像你将一个转换应用于压缩的jpeg图像一样,这个过程也会有所退化.最后的音频最多只会遭受一次压缩循环.

所以这种方法真正的数学答案实际上是没有的.

仅供参考,这里是swift 3中的一些入门代码.需要进一步细化才能跳过文件头.

var outAudioFile:AudioFileID?
var pcm = AudioStreamBasicDescription(mSampleRate: 44100.0,
                                      mFormatID: kAudioFormatLinearPCM,
                                      mFormatFlags: kAudioFormatFlagIsBigEndian | kAudioFormatFlagIsSignedInteger,
                                      mBytesPerPacket: 2,
                                      mFramesPerPacket: 1,
                                      mBytesPerFrame: 2,
                                      mChannelsPerFrame: 1,
                                      mBitsPerChannel: 16,
                                      mReserved: 0)

var theErr = AudioFileCreateWithURL(destUrl as CFURL!,
                                    kAudioFileAIFFType,
                                    &pcm,
                                    .eraseFile,
                                    &outAudioFile)
if noErr == theErr, let outAudioFile = outAudioFile {
    var inAudioFile:AudioFileID?
    theErr = AudioFileOpenURL(sourceUrl as! CFURL, .readPermission, 0, &inAudioFile)

    if noErr == theErr, let inAudioFile = inAudioFile {

        var fileDataSize:UInt64 = 0
        var thePropertySize:UInt32 = UInt32(MemoryLayout<UInt64>.stride)
        theErr = AudioFileGetProperty(inAudioFile,
                                      kAudioFilePropertyAudioDataByteCount,
                                      &thePropertySize,
                                      &fileDataSize)

        if( noErr == theErr) {
            let dataSize:Int64 = Int64(fileDataSize)
            let theData = UnsafeMutableRawPointer.allocate(bytes: Int(dataSize),
                                                           alignedTo: MemoryLayout<UInt8>.alignment)

            var readPoint:Int64 = Int64(dataSize)
            var writePoint:Int64 = 0

            while( readPoint > 0 )
            {
                var bytesToRead = UInt32(2)

                AudioFileReadBytes( inAudioFile, false, readPoint, &bytesToRead, theData)
                AudioFileWriteBytes( outAudioFile, false, writePoint, &bytesToRead, theData)

                writePoint += 2
                readPoint -= 2
            }

            theData.deallocate(bytes: Int(dataSize), alignedTo: MemoryLayout<UInt8>.alignment)

            AudioFileClose(inAudioFile);
            AudioFileClose(outAudioFile);
        }
    }
}